The measurement of the time interval between a start trigger signal and a stop trigger signal is achieved by counting, with a counter, how many times zero-crossing occurs in a reference clock between the timing of an up edge of the start trigger signal (the timing that it rises) and the timing of an up edge of the stop trigger signal. This method, however, provides a resolution no finer than an integer times the period of the reference clock. To obtain a measurement resolution finer than the clock period, various methods have been proposed.
FIG. 20 is an outline configuration diagram of a conventional time measurement device that provides a measurement resolution finer than the clock period, and FIG. 21 is a diagram illustrating the operation of the time measurement device in FIG. 20 (see, for example, Patent Document 1 identified below). In the time measurement device in FIG. 20, a capacitor C is charged to a predetermined voltage in advance. At the timing that a trigger pulse is received, a switch SW is turned on so that the capacitor C is connected to a constant-current circuit to start discharging the electric charge stored in the capacitor C. The discharging is stopped by the switch SW being turned off at the timing that, after the trigger pulse, the reference clock is received (at an up edge timing of the reference clock). The change ΔV in the voltage across the capacitor C resulting from the discharging is read by an AD converter. The amount of integral electric charge that is discharged by the constant-current circuit is proportional to the discharge time, and thus the voltage ΔV has a value commensurate with the time difference ΔT between the trigger pulse and the reference clock. Thus, the voltage ΔV provides information on a time interval shorter than the clock period.